1. Field of the Invention
The present invention relates to an electrolyte solution, a secondary battery using the electrolyte solution, and a secondary battery using a plastisol as a liquid electrolyte.
2. Description of the Related Art
As the market for notebook personal computer, portable telephone, etc. has expanded rapidly, the requirement for batteries used therein, having a high output and excellent stability has increased. To respond to the requirement, there have been developed secondary batteries which use an alkali metal ion (e.g. lithium ion) as a charge carrier and utilize an electrochemical reaction associated with the donation and acceptance of the above ion.
Such batteries using an alkali metal ion need to use a non-aqueous electrolyte solution and, therefore, have had a possibility of reduced battery properties caused by liquid leakage and vaporization. Hence, there have been used, as the solvent for the electrolyte solution, high-boiling basic solvents such as ethylene carbonate, propylene carbonate, diethyl carbonate, dimethyl carbonate, xcex3-butyrolactone and the like, singly or in combination. With these solvents, however, it has been impossible to completely eliminate the possibility of reduced battery properties caused by liquid leakage and/or vaporization. A stable and highly safe electrolyte solution has been required also for electrochemical apparatuses such as electric double layer capacitor, electrolytic capacitor, various sensors and the like; however, no completely satisfactory electrolyte solution has been developed.
Secondary batteries using a liquid electrolyte have, in some cases, a structure in which an active material layer for positive electrode and an active material layer for negative electrode are separated by a separator made of a porous film and the resulting combination of two electrodes and a separator is wound a plurality of times or piled in a plurality of layers. A liquid electrolyte is introduced between the positive electrode and the negative electrode. In such batteries, the film-shaped separator has functions of (1) preventing the contact of two electrode active materials with each other and (2), when, for example, an abnormally large current flows and Joule""s heat is generated, melting and plugging the pores which are the passages of ion. In recent years, as electronic appliances have become smaller and come to possess a higher performance, the secondary batteries used therein have become smaller and come to possess a higher output and a higher capacity; therefore, when short-circuiting arises in the batteries, a large current may be generated and may break the film-shaped separator of battery. In recent years, in response to the requirement for smaller battery, there has come to be often employed a thin battery having such a structure that a combination of a positive electrode active material layer, a negative electrode active material layer and a separator is wound a plurality of times and then crushed. In such a battery, however, the separator receives a large pressure and breaks very easily. The breakage of separator may invite short-circuiting and make charging impossible, or may produce firing or fuming. Therefore, in such a battery, it has been necessary as a measure for possible short-circuiting, to provide a protective circuit or a fuse at the outside of the battery. Thus, in secondary batteries which use a liquid electrolyte and wherein a positive electrode and a negative electrode are separated from each other by a separator film, there have been rooms for improvement, as mentioned above.
Meanwhile, it has been investigated to use a solvent-free polymer solid electrolyte or a polymer gel electrolyte low in solvent content, in order to prevent the reduction in battery properties caused by liquid leakage and vaporization and further prevent the occurrence of short-circuiting and the firing or fuming caused by heat generation. In such a battery constitution, no separator film is required and, therefore, the breakage of separator film and resultant occurrence of short-circuiting, etc. can be eliminated. As the polymer solid electrolyte, there are known those obtained by dissolving a metal salt in a polymer having a polyether segment (e.g. polyethylene oxide) or in a crosslinking product of the polymer.
In U.S. Pat. No. 4,303,748 is disclosed an electricity-generating device of charge and discharge type, which uses, as the electrolyte, a solid solution obtained by dissolving an ionic substance in a polymer having an ethylene oxide main chain. In JP-A-8-7924 is disclosed an ion-conductive polymer obtained by crosslinking a polymer having a polyether segment, with acryloyl group or the like. Further, investigations have been made on polymer gel electrolytes wherein a polymer is allowed to contain an organic solvent for improved ionic conductivity. For example, in JP-B-61-23947 is disclosed a polymer gel electrolyte comprising a polymer (e.g. polyvinylidene fluoride), a group I or II metal salt and an organic solvent having solubility for both the polymer and the metal salt. In U.S. Pat. No. 5,296,318 is disclosed a polymer gel electrolyte obtained by impregnating a hexafluoropropylene-vinylidene fluoride copolymer film with a solution (an organic solvent containing a lithium salt). Also in JP-A-5-109310 is disclosed a method for producing a complex wherein an alkali metal-containing solution is infiltrated into the inside of a crosslinked polymer, by mixing a polymer having a crosslinkable polyether segment, an alkali metal salt and a solvent capable of dissolving the metal salt, molding the mixture, and applying a light, a radiation or the like to the molded material to give rise to crosslinking. Investigations have also been made on polymer gel electrolytes using a combination of two or more kinds of polymers. For example, in JP-A-58-75779 is disclosed a battery constituted by at least one kind of polymer selected from a polyvinylidene fluoride, a polymethyl methacrylate and other particular polymers, a lithium salt, a particular organic solvent, a metal lithium negative electrode and a positive electrode consisting of a particular inorganic compound. In JP-A-9-971618 is disclosed a polymer gel electrolyte obtained by preparing a mixture or solution of a polymer sparingly soluble in an organic electrolytic solution and a polymer soluble in the organic electrolytic solution, making the mixture or solution into a polymer alloy film, and impregnating the film with the organic electrolytic solution to give rise to gelation. Therein are shown, as an example of the polymer sparingly soluble in the organic electrolytic solution, a polyvinylidene fluoride and, as an example of the polymer soluble in the organic electrolytic solution, a polyethylene oxide. In these polymer solid electrolytes and polymer gel electrolytes, however, as compared with liquid electrolytes, ionic conductivity is low and it is difficult to obtain a high output.
As mentioned above, with a liquid electrolyte, although a high ionic conductivity is obtained, it is difficult to completely eliminate the possible liquid leakage and vaporization from the very small flaws of sealed container. Therefore, in batteries which use an alkali metal ion as a charge carrier and wherein a positive electrode and a negative electrode are adjacent via an electrolytic solution, it has been impossible to completely eliminate the possible reduction in battery properties, caused by liquid leakage and vaporization; further, there has been a risk that the separator film breaks easily and short-circuiting takes place between the positive electrode and the negative electrode, making charging impossible and inducing firing or fuming.
Meanwhile, with polymer solid electrolytes containing no solvent or with polymer gel electrolytes containing a solvent in a low concentration, although the risk of short-circuiting is low, no sufficiently high ionic conductivity is obtained, making it difficult to obtain a secondary battery of high output.
In view of the above situation, it is an object of the present invention to provide a secondary battery which is free from liquid leakage and vaporization, maintains sufficiently high ionic conductivity, hardly causes short-circuiting or the like, and is highly stable.
According to the present invention, there is provided an electrolyte solution consisting of a basic solvent containing an alkali metal salt and a halogenated polyolefin both dissolved therein.
According to the present invention, it is possible to obtain an electrolyte solution high in ionic conductivity and excellent in stability and safety. Containing an alkali metal salt and a halogenated polyolefin both dissolved, the present electrolyte solution is substantially free from liquid leakage or vaporization and has high ionic conductivity.
According to the present invention, there is also provided a secondary battery using an alkali metal ion as a charge carrier and having a structure in which a positive electrode and a negative electrode are adjacent to each other via an electrolyte solution, in which secondary battery the electrolyte solution consists of a basic solvent containing an alkali metal salt and a halogenated polyolefin both dissolved therein.
Using the above-mentioned electrolyte solution of the present invention, the above secondary battery has a high output density and high safety.
According to the present invention, there is further provided a secondary battery having a structure in which a positive electrode layer and a negative electrode layer are laminated via a separator and a liquid electrolyte is allowed to be present between the positive electrode layer and the negative electrode layer, in which secondary battery the liquid electrolyte is a plastisol containing an electrolyte salt.
According to the present invention, there is also provided a process for producing a secondary battery, which comprises:
a step of laminating a positive electrode layer and a negative electrode layer via a separator,
a step of introducing a plastisol containing an electrolyte salt, between the positive electrode layer and the negative electrode layer,
a step of applying a voltage between the positive electrode layer and the negative electrode layer to heat part of the plastisol, and
a step of cooling the plastisol.
The above secondary battery is characterized in that it uses a plastisol as a liquid electrolyte. xe2x80x9cPlastisolxe2x80x9d refers to a paste-like sol having fluidity, obtained by dispersing a thermoplastic resin powder in a plasticizer, as defined in, for example, xe2x80x9cNew Polymer Dictionary (edited by Polymer Dictionary-Editing Committee of The Society of Polymer Science, Japan, published from Asakura Shoten in 1988)xe2x80x9d. In the plastisol, the most part of the thermoplastic resin powder is not dissolved and is dispersed in the plasticizer. When the plastisol is heated to a certain temperature or higher, the thermoplastic resin powder dissolves in the plasticizer and, when the plastisol is then cooled, a polymer gel is formed. This unique property of plastisol is utilized in the present invention.
Being a liquid electrolyte, the plastisol has high ionic conductivity as compared with a gel or solid electrolyte. As mentioned previously, in secondary batteries using a conventional liquid electrolyte, Joule""s heat is generated when an abnormally large current flows inside the battery, causing separator"" breakage, etc. In contrast, in the secondary battery of the present invention using a plastisol which becomes a gel upon generation of Joule""s heat, a polymer gel is formed at the sites where an abnormally large current flows. By the formation of this polymer gel, the sites where breakage tends to occur, are reinforced, and the sites already having breakage are automatically repaired; and the short-circuiting inside battery can be effectively prevented when an abnormally large current appears. Thus, in spite of using a liquid electrolyte, good stability and good safety can be realized in the present secondary battery.
Moreover, since the plastisol has a low vapor pressure and a high viscosity as compared with ordinary liquid electrolytes, there is neither leakage nor vaporization of electrolytic solution; therefore, from this point as well can be achieved improvement in stability and safety.
The process for production of secondary battery according to the present invention is characterized in that it uses a plastisol as a liquid electrolyte and has a step of applying a voltage between a positive electrode layer and a negative electrode layer to heat part of the plastisol. In a state that a voltage is applied between the electrodes, a current density distribution appears in the separator, microscopically speaking. The sites of high current density correspond to sites which easily break during the use of battery; at such sites, Joule""s heat appears and the plastisol becomes a solution. Hence, by conducting cooling after the above step, a polymer gel is formed selectively at the above sites and reinforcement of the sites is made. In this case, only part of the plastisol becomes a polymer gel and the most part of the plastisol maintains a sol form and constitutes the electrolyte of secondary battery. The plastisol constituting the electrolyte of secondary battery functions, as mentioned previously, so as to prevent the breakage of separator when an abnormally large current flows.
Thus, the secondary battery produced by the process of the present invention, using a plastisol as an electrolyte has not only a self-repairing function but also a function of beforehand reinforcing sites of separator which may easily break, and can effectively prevent the breakage of separator which may occur when an abnormally large current flows.